This invention relates in general to a process for welding flexible belts and more specifically to a process for welding belts using a variable welding rate.
Various techniques have been devised to form belts from webs. Thermoplastic webs may be joined by overlapping the edge of one web over another and placing the overlapped or seam portion of the webs over a base beneath a vibrating welding element. The vibrating welding element may be a horn vibrating at an ultrasonic frequency while it is brought into forced contact with one side of the overlapped webs while the other side of the overlapped web seam is supported on an anvil surface. Transfer of vibratory energy from the horn to the web material is effected upon impact of a suitably shaped surface of the horn on the seam of the web material. The vibrating energy causes surface heat to be generated between the overlapping webs along the area of contiguous contact between the webs. The horn normally resonates toward and away from the seam at a frequency of about 16 kHz or higher, e.g. 20,000 kilohertz. A typical welding horn amplitude at a frequency of about 20,000 kilohertz is about 80 micrometers and a typical welding horn traverse rate is between about 4 centimeters per second and about 7 centimeters per second. Thus, for example, a traverse speed of about 5 centimeters per second is described in U.S. Pat. No. 4,838,964. The weld may be in the form of a chain of spots along the seam or a continuous weld. The technique of ultrasonic welding of thermoplastic material is well known and illustrated, for example, in U.S. Pat. Nos. 4,532,166, 4,838,964, 3,879,256, 3,939,033, 3,947,307 and 3,459,610, all of these patents being incorporated by reference herein in their entirety.
Acceptable welds for forming belt shaped electrophotographic imaging members have been obtained which perform satisfactorily when transported around relatively large diameter rollers. The welded seams of these belts contain a deposit of web material melted during welding. These deposits or "weld splashes" are formed on each side of the welded web adjacent to an adhering to each end of the original web and to the regions of the web underlying each splash. Unfortunately, when the belts are transported around very small diameter rollers having, for example a diameter of about 19 mm, the weld splash on the outer surface of the belt gradually separates from the upper end of the photoreceptor web during cycling to form an open crevasse or crack which is repeatedly struck by the cleaning blade during image cycling to cause the weld splash on the outer surface of the belt to also separate from the underlying web so that it is held to the belt at a small linear region located along the length of the splash on the side of the splash opposite the upper end of the photoreceptor web. This small linear region located along the length of the splash on the side of the splash opposite the upper end of the photoreceptor web functions as a hinge that allows the weld splash to pivot or flop away from the upper end of the photoreceptor web and from the underlying web so that carrier beads and toner particles collected in the space between the web and splash are periodically ejected when the splash flops open and closed when the seam passes the cleaning blade and also when it travels around small diameter rollers. The ejected material is carried to various subassemblies (e.g. corotrons, lamps) and causes them to fail and ultimately cause copy defects. Collisions of the blade with the flopping splash results in the blade becoming chipped and pitted. These chips and pits in the blade leave streaks of toner on the photoreceptor surface which eventually appear as streaks on copies made during subsequent electrophotographic imaging cycles and adversely affect image quality.
It has been discovered that splash separation in the photoreceptor during cleaning and passage around very small diameter rollers can be greatly minimized by welding the seams at very high horn oscillation frequencies of, for example, about 40 kilohertz and lower horn amplitudes of between about 60 micrometers and about 75 micrometers. This technique is described in a patent application entitled "HIGH FREQUENCY WELDING OF THERMOPLASTIC BELTS", filed in the names of Karl V. Thomsen et al on the same day as the instant application. The entire disclosure of the Karl V. Thomsen et al patent application is incorporated herein by reference. When seams are welded with welding horns operating at this high frequency and lower amplitude, the weld can occasionally be difficult to initiate. More specifically, the horn will occasionally lift up the top layer of the overlapped photoreceptor edges. The lifting of the top layer can cause pressure and heat transfer to the bottom layer to be unstable and a raised ridge can form in the thermoplastic material in the top layer on the overlapped photoreceptor seam in the region initially contacted with the moving welding horn. This raised ridge is struck by the doctor blade during the cleaning step of electrophotographic imaging cycles and eventually forms a notch in the edge of the cleaning blade which causes streaks to print out on every copy during electrophotographic image cycles. Belts having the raised ridge at the seam must be scrapped and the belt fabrication line must be shut down to allow readjustment of the welding station to reduce the number of defective belts formed.